The present invention relates to a method and an apparatus for forming a coating of a thin, uniform film of viscous liquid, by using a small amount of the liquid, on a surface of an object having a comparatively large surface area.
Conventionally, as the methods for the formation of a film of a resist, such as a pigment-dispersed resist or a water soluble resist on rectangular substrates and the like having a relatively large size (150 mm.times.150 mm or larger) for use in color image sensors and color filters for LCD, there have been known a spin coating method in which centrifugal force is used to disperse a resist over the entire flat surface of the substrate, and a roll coating method in which a roll is used to transfer the resist.
In the spin coating method, the substrate is held by suction against a spinner chuck. Then viscous liquid is dropped over the upper surface of the substrate which is to be coated, while the substrate is rotated, so that centrifugal force causes the viscous liquid to spread over the top surface of the substrate.
In this method, the substrate is held by suction against the spinner chuck and the places where the suction force is exerted are thereby deformed downwards so that the thickness of the applied layer is thicker at such places to cause unevenness of the coating. This is not desirable for the reasons to be described later. In addition, in this method, there is formed a "fringe" where there is an excess thickness of the viscous liquid along the rim. This is also not desirable for the reasons to be described later. Furthermore, in this spin coating method, the amount of the drops of viscous liquid that is actually used to form the layer is no more than 2% to 3% and so the method is uneconomical.
On the other hand, in the known roll coating method, the viscous liquid consumption is less than that in the spin coating method, but unevenness in the shape of lines occurs in the direction parallel to the direction of movement of the roll, so that the substrate to which the viscous liquid has been applied cannot be used as a filter for high-quality image displays.
Japanese Patent Laid-Open Pub. No. 63-246820 discloses a method wherein for the application of a photo-sensitive resin to a flat plate object, a roll coater is used to apply the resin, and then the flat object is rotated at a predetermined speed to spread the photo-sensitive resin.
In Japanese Patent Laid-Open Pub. No. 63-313159 there is disclosed an apparatus which comprises: a holding device that holds a substrate horizontally so as to be freely rotatable; a resist application device having a transfer roll for applying a resist, a moving means for moving the resist application device and its transfer roll portion relative to the surface of the substrate to which the resist is to be applied; and a rotating device to rotate the holding device and thereby the substrate to which the resin has been applied.
In these method and apparatus, the resin is applied by the roll coating method prior to the use of the spin coating method, but the line unevenness that occurs with the roll coating method become fixed when the roll coating has completed and cannot be smoothed even if the spin coating method is carried out thereafter. This is the same regardless of whatever resist is coated to whatever thickness. In particular, in the case of water soluble resists that have a low viscosity and a poor wettability with respect to the substrate, it is not possible to use methods that include the roll coating to apply such resists.